The proposed research is planned to study the functional development of sympathetc innervation of the heart directly in puppies and indirectly in newborn infants. Experiments are carried out in open chest newborn puppies up to 6 weeks of age. After bilateral dissection of the sympathetic cardiac nerves, refractory period changes are determined during maximal stimulation of the individual nerves. The findings from different age groups including adult dogs are compared and functional maturational changes as well as differences between newborn and adult dogs are found. Serial electrocardiograms are obtained in full term premature infants during the first three months of life, starting 24 hours after birth. The data will be processed through a computer and any evidence of sympathetic nerve imbalance such as ST-T waveform abnormalities and tachyarrhythmias will be identified, followed and correlated with maturation. Finally, the results of the electrocardiographic studies in newborns will be correlated with results of the animal studies. BIBLIOGRAPHIC REFERENCES: Kralios, F.A., Martin, L., Burgess, M. J., and Millar, K.: Local ventricular repolariztion changes due to sympathetic nerve branch stimulation. Amer. J. Physiol. 228-5: 1621-1626, 1975. Millar, K. and Kralios, F.A.: Reflexly induced neurogenic T wave changes in dogs; left sympathetic predominance. Circulation, 52-II:63, 1975. (Abstract).